Method of preparing photographic silver halide emulsions

ABSTRACT

A method is disclosed of preparing washed chemically sensitized silver halide emulsions according to which washing occurs by coagulation based on the use of acid-coagulable gelatin derivatives, wherein between coagulation washing and chemical sensitization the pH of the emulsion is raised to a value above 8. By this pH raise it is possible to increase the speed, gradation and maximum density.

111mm States Patent 1 1 1 1 3,873,322 llorens 1 Mar. 25, 1975 METHOD OF PREPARING 2,614,929 10/1952 Yutzy et a1. 96/1148 PHOTOGRAPH: SILVER HALIDE 3,085,897 4/1963 Priest et al 1 17/34 3,782,954 l/l974 Porter et al. 96/94 R EMULSIONS Inventor: Raymond Leopold Florens, Edegem,

Belgium Assignee: Agfa-Gevaert, Mortsel, Belgium Filed: May 11, 1973 Appl, No.1 359,512

Foreign Application Priority Data May 12, 1972 United Kingdom 22433/72 [1.8. CI 96/ll4.8, 96/94 R, 96/114, 96/1 14.7

Int. Cl G03c l/02 Field of Search 96/1148, 94 R, 114, 114.7; 117/34 References Cited UNITED STATES PATENTS 10/1952 Yutzy et a1. 96/1148 Primary Examiner-Norman G. Torchin Assistant Examiner-Alfonso T. Suro Pico Attorney, Agent, or Firm-A. W. Breiner [57] ABSTRACT A method is disclosed of preparing washed chemically sensitized silver halide emulsions according to which washing occurs by coagulation based on the use of acid-coagulable gelatin derivatives, wherein between coagulation washing and chemical sensitization the pH of the emulsion is raised to a value above 8. By this pH raise it is possible to increase the speed, gradation and maximum density.

10 Claims, N0 Drawings METHOD OF PREPARING PHOTOGRAPHIC SILVER HALIDE EMULSIONS The present invention relates to the preparation of photographic gelatino silver halide emulsions.

The preparation of a silver halide emulsion basically consists of a number of stages which are as follows:

1. the precipitation of very small silver halide grains, called emulsification, by mixing an aqueous solution of a water-soluble silver salt commonly silver nitrate with an aqueous solution of a water-soluble halide commonly an ammonium halide or alkali metal halide in the presence ofa hydrophilic colloid, in particular gelatin, which may be dissolved in either or both of the above solutions or in a separate solution;

2. the growth of the grains to the appropriate size, called physical ripening;

3. the removal of the by-products from the grain formation and growth stage, called washing,

4. the sensitization of the silver halide grains to obtain the desired speed, called digestion or chemical ripening or chemical sensitization, and

5. the final preparation including the addition of spectral sensitizers (if desired) and the addition of other conventional emulsion ingredients before coatmg.

At the conclusion of the physical ripening stage the photographic emulsion comprises by-products of the grain formation and growth stage, excess halide, ammonia or acid, etc. which should be removed by washing so that at the chemical ripening or digestion stage a practically neutral (pH from about 6 to about 7) silver halide emulsion is obtained which is substantially free from undesirable water-soluble compounds.

The most conventional method of washing consists in cooling the emulsion so that it sets to a fairly soft jelly, comminuting it into small fragments, usually by shredding to produce noodles, and washing the fragments by suspending them in water.

The water has to be changed frequently or continuously until the emulsion fragments are sufficiently free from the undesirable products. This washing method is very time consuming and allows no opportunity for concentrating the emulsion as the emulsion fragments swell during washing and the viscosity drops.

Modern methods of emulsion washing are based on the principle of causing the gelatin to coagulate or flocculate so that it settles together with the silver halide and thus separates from the aqueous solution comprising the soluble by-products. The supernatant solution is decanted or removed by some other method and the settled gelatin-silver halide mixture is redispersed, after washing of the precipitate, in water or aqueous gelatin. The flocculation method offers a number of advantages; in particular, a very high percentage of the unwanted salts may be removed in one stage and concentrated silver halide emulsions can be prepared.

Various methods of coagulating or flocculating silver halide emulsions have been proposed. For example, the gelatin-silver halide mixture can be coagulated by addition of salts e.g. sodium and ammonium sulphate. However, a large amount of the salt is required to obtain complete flocculation and this salt should be removed by repeated washing. Moreover, in order to avoid swelling of the emulsion during washing an additional amount of salt is added in order to harden the emulsion which may make it difficult to redisperse the coagulum.

Water-miscible organic solvents into which the gelatin is insoluble can also be used to bring about flocculation, but this method is expensive and the precipitate contains salts that are insoluble in the organic solvent used.

It has further been proposed to use as flocculating agents organic polymeric or non-polymeric sulphonic acids or sulphuric acids for example naphthalene disulphonic acids, polystyrene sulphonic acid and derivatives, long-chain alkyl sulphonic acids and sulphuric acids e.g. the well-known anionic wetting agents, etc. Disadvantages of these methods are the fact that if relatively high concentrations of the sulphonic acids or sulphates (20 to 30 are used they have an adverse effect on the coating properties of the emulsion whereas if the said flocculating agents are used in low concentration the coagulation times are rather long and there is a tendency for the coagulum to swell and to become redispersed when it is washed. Moreover the coagulation has to be effected at low pl-l-values and it is known from emulsion preparation practice that in the case of highly sensitive ammonia emulsions, such as are used in the preparation of X-ray films, this unavoidable depression of the pH value brings about a loss in sensitivity of the emulsion.

Very convenient coagulation or flocculation methods for removing undesirable reaction products, residual water-soluble salts and excess water are based on the use of acid-coagulable gelatin derivatives e.g. as described in US. Pat. Nos. 2,614,928 of Henry C. Yutze and Gordon F. Frame, issued Oct. 21, 1952, 2,614,929 of Henry C. Yutze and Frederik J. Russell, issued Oct. 21, 1952 and 2,728,662 of Henry C. Yutze and Gordon F. Frame, issued Dec. 27, 1955. These acid-coagulable gelatin derivatives are reaction products of gelatin with organic carboxylic or sulphonic acid chlorides, carboxylic acid anhydrides, aromatic isocyanates or 1:4- diketones. According to the procedures described in the above United States Patent Specifications the silver halide may be precipitated by reaction of a watersoluble silver salt solution and a water-soluble halide solution in an aqueous solution of the acid coagulable gelatin derivative or in an aqueous solution of gelatin to which an acid coagulable gelatin derivative is added in sufficient proportion to impart acid-coagulable properties to the entire mass. Alternatively, the silver halide may be precipitated in an aqueous solution of gelatin whereupon the dispersion is treated optionally after physical ripening with an organic reagent e.g. an organic acid chloride or anhydride, an aromatic isocyanate or a 1:4-diketone, in the presence of alkali so as to form acid-coagulable gelatin derivative in situ. To effect coagulation or flocculation, the dispersion comprising silver halide and gelatin derivative is acidified to a pH of approximately 4.5 or less, e.g. between 3 and 4.5. The coagulum formed may be separated from the liquid by several techniques e.g. by decanting the supernatant liquid followed by washing of the coagulum. The washed coagulum is then redispersed to form photographic emulsions for the subsequent finishing and coating operations.

Although the coagulation methods based on the use of gelatin derivatives are attractive on the grounds that complete coagulation is easy to effect, that the number of washing operations can be reduced and that silver halide emulsions with high silver halide concentration can be prepared, they cause an undesirable loss in photographic speed owing to the low pH at which coagulation is effected so that these methods do not permit the preparation of highly sensitive silver halide emulsions.

It has now been found that the loss in speed caused by coagulation based on the use of acid-coagulable gelatin derivatives can be substantially restored by raising the pH to a value above 8 some time between the conclusion of coagulation washing and the beginning of chemical ripening. This procedure of raising the pH also has a favourable effect on the gradation and maximum density.

The present invention, therefore, provides a method of preparing photographic silver halide emulsions in which an emulsion incorporating an acid-coagulable gelatin derivative is prepared and wherein the undesirable byproducts of silver halide grain formation and physical ripening are removed by lowering the pH (e.g. to a value in the range from about 3 to about 4.5) to bring about coagulation and by washing the coagulum and the coagulum is redispersed and the silver halide emulsion formed is subsequently chemically sensitized, the method being characterized in that some time between the conclusion of the washing operation and the chemical sensitization the pH of the silver halide emulsion is raised to a value above 8.

As already referred to above coagulation washing of silver halide emulsions based on the use of acidcoagulable gelatin derivatives is well known in the art of silver halide emulsion preparation. The silver halide is preferably precipitated in an aqueous solution of the gelatin derivative itself or in an aqueous solution of both normal gelatin and a gelatin derivative the said derivative being present in a proportion sufficient to impart acid-coagulable properties to the entire mass. Alternatively, the gelatin derivative may be added after the stage of emulsification in normal gelatin, and even after the physical ripening stage, provided it is added in an amount sufficient to render the whole coagulable under acid conditions. As a further alternative the precipitation may occur in an aqueous solution of gelatin and an acid coagulable gelatin derivative may be formed in situ by adding an organic reagent as hereinbefore referred to before or after physical ripening. Examples of acid-coagulable gelatin derivatives suitable for use in accordance with the present invention can be found e.g. in the United States Patent Specifications referred to above. Particularly suitable are phthaloyl gelatin and N-phenylcarbamoyl gelatin.

After the emulsification and physical ripening stage, the silver halide emulsion comprising acid-coagulable gelatin derivative is acidified e.g. by means of dilute sulphuric acid, citric acid, acetic acid, etc. (e.g. to a pH within the range of about 3 to about 4.5) so as to effect coagulation. The coagulum formed may be removed from the liquid by any suitable means, for example the supernatant liquid is decanted or removed by means of a siphon, whereupon the coagulum is washed out once or several times. Whereas in the preceding emulsification and physical ripening stages a temperature is used generally comprised between about 35C and about 70C, dependent on the desired grain size and thus emulsion speed to be obtained, coagulation may occur at a temperature comprised between about 10C and about 50C. It is preferred to lower the temperature to about lO30C before coagulation and thus before adjusting the pH to the coagulation point.

Washing of the coagulum may occur by rinsing with mere cold water. However, the first wash water is preferably acidified to lower the pH of the water to the pH of the coagulation point. Alternatively washing may be effected by redispersing the coagulum in water at elevated temperature using a small amount of alkali e.g. sodium or ammonium hydroxide, recoagulating by addition of an acid to reduce the pH to the coagulation point and subsequently removing the supernatant liquid. This redispersion and recoagulation operation may be repeated as many times as is necessary. As a third method of washing the coagulum is redispersed in water at a pH below the coagulum point by addition of an acid whereupon recoagulation is effected by addition of alkali to raise the pH to the coagulation point and the supernatant liquid is removed.

After the washing operation, the coagulum is redispersed to form a photographic emulsion suitable for the subsequent finishing and coating operations by treating, preferably at a temperature within the range of about 35 to about 60C, with the required quantity of water, normal gelatin and, if necessary, alkali for a time sufficient to effect a complete redispersal of the coagulum. Instead or in addition to normal gelatin, which is preferably used, other known photographic hydrophilic colloids can also be used for redispersion e.g. a gelatin derivative as referred to above, albumin, agar-agar, sodium alginate, hydrolysed cellulose esters, polyvinyl alcohol, hydrophilic polyvinyl copolymers, etc.

In accordance with the present invention, the coagulation washed silver halide emulsion is alkalized to a pH value above 8, in the stage between the last washing of the coagulated silver halide emulsion and the chemical sensitization i.e. before, during or after redispersion of the coagulated silver halide emulsion. Thus, redispersion may be effected at a pH-value above 8, or, preferably, after redispersion of the silver halide emulsion, the pH is adjusted to a value above 8.

Though the silver halide emulsion, after having raised the pH above 8, may be neutralized (pH from about 6 to about 7) immediately, the emulsion is generally kept at the alkaline pH for some minutes e.g. from 5 to 30 minutes before neutralisation whereupon it is digested to the optimum relation between fog and sensitivity in a known manner at a temperature generally comprised between about 35C and about 60C.

Thus, according to a preferred embodiment of the present invention the method for preparing photographic silver halide emulsions comprises the following steps:

1. precipitation of the silver halide grains by reaction of silver nitrate with an ammonium or alkali metal halide in an aqueous solution of an acid-coaguable gelatinderivative, preferably phthaloylgelatin, or a mixture of normal gelatin with a said gelatin-derivative,

2. physical ripening of the silver halide grains,

3. coagulation by lowering the pH e.g. to a value in the range of about 3 to about 4.5,

4. removal of the supernatant liquid and washing of the coagulum to the desired degree by means of cold water; the pH of the first wash-water may be adjusted to the pH of the coagulum point,

5. redispersion of the washed coagulum with addition of water and gelatin,

6. raising the pH to a value above 8 by addition of alkali e.g. ammonium or sodium hydroxide,

7. adjusting the pH to approximately neutral, say a pH within the range from about 6 to about 7, and

8. chemical sensitization of the emulsion to the optimum fog-sensitivity relation.

Digestion or chemical sensitization of the silver halide emulsion may occur according to any of the accepted procedures e.g. as described on page 107 of the December 1971 issue of Product Licensing lndex published by Industrial Opportunities Limited, Havant, England and in the patent literature referred to therein. It may be digested in the presence of small amounts of sulphur-containing compounds such as allyl thiocyanate, allyl thiourea, sodium thiosulphate, etc. The emulsion may also be sensitized by means of reductors for instance tin compounds as described in United Kingdom Pat. Specification No. 789,823 filed Apr. 29, 1955 by Gevaert Photo-Producten N.V., polyamines e.g. diethyltriamine, and small amounts of noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium as described by R. Koslowsky, Z. Wiss. Phot. 46, 67-72 (1951). Of course the emulsions may also be chemically sensitized by the combined use of these chemical sensitizers.

Photographic silver halide emulsions of all kinds may be prepared by the process of the invention, which include negative as well as direct-positive silver halide emulsions, coarse-grain as well as fine-grain silver halide emulsions. The silver halides may be silver bromide, silver chloride or silver chlorobromide and may comprise a small amount up to of silver iodide.

Raising the pH to a value above 8 at a stage of emulsion preparation intermediate the conclusion of coagulation washing and the chemical ripening results in an increase of the speed and gamma as well as maximum density, which makes the method of the invention of particular importance in the preparation of highly sensitive ammonia emulsions e.g. silver bromoiodide ammonia emulsion as used in the preparation of X-ray films. Moreover, the method of the present invention permits to markedly reduce noble metal sensitization. Before coating on a support, any one or more of the common so-called coating finals may be added to the photographic silver halide emulsions prepared in accordance with the present invention. These coating finals include spectral sensitizers, colour couplers, antifoggants and emulsion stabilizers, coating aids, plasticizers, light-absorbing dyes, hardeners, development modifiers, etc. a survey of which can be found on pages 107-109 of the December 1971 issue of Product Licensing lndex, published by Industrial Opportunities Limited, Havant, England.

The emulsions may comprise lactams e.g. caprolactam in order to reduce possible pressure desensitization.

The silver halide emulsions prepared in accordance with the present invention may be coated on the wide variety of supports known for use in photographic silver halide elements which include cellulose nitrate film, cellulose acetate film, poly(vinyl acetal) film, polystyrene film, po|y(ethylene terephthalate) film, polycarbonate film and related films or resinous materials, as well as glass, paper, metal and the like. Paper supports may be used which are partially acylated or coated with baryta and/or an a-olefin polymer, particularly a polymer of an a-olefin containing from 2 to 10 C-atoms such as polyethylene, polypropylene, ethylenebutylene copolymers and the like.

The following examples illustrate the present invention.

EXAMPLE 1 High-speed gelatino silver iodobromide X-ray emulsions comprising 98 mole of silver bromide and 2 mole of silver iodide were prepared in the following way.

Emulsion A a. The silver halide was precipitated in aqueous phthaloyl gelatin (amino groups of gelatin acylated for about 90 to I00 percent) by addition of an aqueous solution of halide and an ammoniacal aqueous solution of silver nitrate at a temperature of 44C. The silver halide grains were physically ripened at 44C in a conventional way until the desired grain-size, grain size distribution and grain sensitivity was reached.

b. At the end of the physical ripening stage, the emulsion was coagulated by adjusting the pH of the emulsion to 3.6 by means of sulphuric acid. The silver halide emulsion settled rapidly and the supernatant liquid was decanted.

c. Washing occurred by addition of cold water (l0-l5C) to which citric acid was added to adjust the pH to 3.5. The emulsion was agitated to leach out the water-soluble salts as much as possible, whereupon the emulsion was allowed to settle and the supernatant liquid was decanted again. This washing procedure was repeated, using now however mere cold water, until the emulsion was sufficiently free from soluble salts.

d. After washing, the coagulated silver halide emulsion was redispersed in water "and sufficient inert gelatin was added to obtain a ratio of gelatin to silver halide (expressed as silver nitrate) of 0.4. The pl-l-value was adjusted to 6.5 and the pAg was adjusted to a value corresponding to an E.M.F. of+ mV (Ag/saturated calomel electrode).

e. The emulsion obtained was chemically sensitized by addition of a sulphur and gold sensitizer and digesting at 47C until the optimum sensitivity-fog relation was reached.

Emulsion B Emulsion B was prepared in the same way as emulsion A with the difference however that:

the physically ripened emulsion was cooled to 25C before coagulation at pH 3.6,

the pH-value of the redispersed emulsion was adjusted to 9.2 by means of sodium hydroxide and kept at this value for 10 minutes whereupon the pH-value was adjusted to 6.5 by means of citric acid and the pAg was adjusted to a value corresponding with an E.M.F. of 70 mV (Ag/saturated calomel electrode),

the emulsion obtained was chemically sensitized by addition of a sulphur and gold sensitizer and digestion at 47C until the optimum sensitivity-fog relation was reached. As compared with emulsion A, in order to reach optimum sensitivity, the amount of noble metal sensitizer (potassium aurithiocyanate) had to be reduced markedly i.e. from 6.7 mg to 2.4 mg for an amount of silver halide corresponding to 500 g of silver nitrate.

After addition of antifoggants, emulsion stabilizers, coating aids and hardeners, the emulsions A and B were coated under identical circumstances on a polyethylene terephthalate support and overcoated with a gelatin antistress layer comprising formaldehyde as hardening agent.

The values of fog (F), gradation (G), speed (S) and maximum density (D) of the materials A and B formed, were determined after development for 4 min. at 20C in a developer of the following composition:

water 800 ml p-monomethyl aminophenol sulphate 4 g sodium sulphite (anhydrous) 65 g hydroquinone l g sodium carbonate (anhydrous) 45 g potassium bromide g water to make 1000 ml that at the end of the physical ripening, the emulsion was not cooled to C before coagulating at pH 3.6.

Emulsion D Table Mate- 4 min. development 23 sec. development rial F o 5, 5 D F 0 5 5 D B 0.06 2.86 I00 100 2.95 0.09 2.66 100 I00 3.05 C 0.05 2.72 89 87 2.78 0.09 2.69 80 89 2.98 D 0.05 2.03 74 60 2.45 0.06 1.85 71 62 2.65

and after processing in an automatic 90 seconds pro- 5 The above results show that raising the pH between cessing machine in which development occurred for 23 seconds at C in Agfa-Gevaerts hardening developer G 138 which comprises hydroquinone and l-phenyl-3- pyrazolidinone as developing agents and glutaraldehyde as hardener.

The results attained are listed in the following table. The speed values were measured at density 0.1 above fog (S,) and 1 above fog (S they are relative values with respect to the speed of emulsion A for which a value 100 has been given.

redispersion of the coagulated emulsion and the chemical ripening results in an increase of the gradation, speed and maximum density (compare results of B with those of D). They further show that the increase in gradation, speed and maximum density is higher when reducing the temperature before coagulation (compare results of B with those of C).

EXAMPLE 3 An e rnulsionBwas prepared in exactly the same way Table Mate- 4 min. development 23 sec. development rial F G S S D F G S, S D

A 0.01 3.49 100 100 3.20 0.03 3.14 I00 100 3.45 B 0.06 2.86 181 170 2.95 0.09 2.66 204 166 3.05

It should be noted that notwithstanding the low fog as emulsion B of example 1 except for the chemical ripvalues of emulsion A, this emulsion was completely ripened i.e. further ripening could not increase the speed.

This emulsion was prepared in exactly the same way as emulsion B of example 1.

Emulsion C This emulsion was prepared in exactly the same way as emulsion B of example 1 with the only difference.

ening. After having adjusted the pAg to a value corresponding with an E.M.F. of+ mV, the emulsion was divided into two aliquot portions B and B Emulsion B was chemically sensitized as described for emulsion B of example 1 (2.4 mg of potassium aurithiocyanate per amount of silver halide corresponding to 500 g of silver nitrate) whereas emulsion B was chemically sensitized as described for emulsion A of example 1 (6.7 mg of potassium aurithiocyanate per amount of silver halide corresponding to 500 g of silver nitrate).

Emulsions B and B were further treated as described in Example 1.

The results attained are listed in the following table.

Table Mate 4 min. development 23 sec. development rial F G S, S D F 6 S S D B 0.05 3.14 I00 3.40 0.09 3.0] 100 lOO 3.50 B 0.01 3.61 5l 54 3.25 0.05 3.40 49 55 350 EXAMPLE 4 Example 1 was repeated with the only difference that now N-phenylcarbamoyl gelatin was used instead of phthaloyl gelatin.

The results attained after 90 sec. processing were as follows.

Material Fog Gradation Speed 1 Speed 2 D We claim:

1. In a method of preparing a photographic silver hal ide emulsion in which an emulsion incorporating an acid coagulable gelatin derivative is prepared and wherein the undesirable by-products of silver halide grain formation and physical ripening are removed by lowering the pH to bring about coagulation and by washing the coagulum, and the coagulum is redispersed and the silver halide emulsion formed is subsequently chemically sensitized, the improvement which comprises raising the pH of the silver halide emulsion to a value above 8 some time between the conclusion of the washing operation and the chemical sensitization.

2. Method according to claim 1, wherein the said gelatin-derivative is phthaloyl gelatin.

3. Method according to claim 1, wherein the silver halide grain formation occurs in an aqueous solution of the said gelatin-derivative.

4. Method according to claim 1, wherein the washed coagulum is redispersed under addition of water and normal gelatin.

5. Method according to claim 1, wherein the pH of the silver halide emulsion is raised to a value above 8 after having redispersed the washed coagulum.

6. Method according to claim 1, wherein immediately before coagulation the temperature of the physically ripened emulsion is adjusted to a value between about 10 and about 30C.

7. Method according to claim 1, wherein the coagulum is washed at least once with cold water.

8. Method according to claim 7, wherein the pH of the first wash water is adjusted to the pH at which coagulation is effected.

9. Method according to claim 1, wherein the silver halide emulsion is a highly sensitive ammoniacal silver bromiodide emulsion.

10. Method according to claim 1, comprising the steps of:

1. precipitation of the silver halide grains in an aqueous solution of the said acid-coagulable gelatin derivative or a mixture of normal gelatin and an acidcoagulable gelatin derivative,

2. physical ripening of the silver halide grains,

3. lowering the pH to bring about coagulation,

4. removal of the supernatant liquid and washing of the coagulum,

5. redispersion of the washed coagulum under addition of water and gelatin,

6. raising the pH of the redispersed coagulum to a value above 8 and keeping the emulsion at this value for some minutes,

7. adjusting the pH to a value between about 6 and about 7,

8. chemical sensitization of the emulsion to the opti- 

1. IN A METHOD OF PREPARING A PHOTOGRAPHIC SILVER HALIDE EMULSION IN WHICH AN EMULSION INCORPORATING AN ACID COAGULABLE GELATIN DERIVATIVE IS PREPARED AND WHEREIN THE UNDERSIRABLE BY-PRODUCTS OF SILVER HALIDE GRAIN FORMATION AND PHYSICAL RIPENING AE REMOVED BY LOWERING THE PH TO BRING ABOUT COAGULATION AND BY WASHING THE COAGULUM, AND THE COAGULUM IS REDISPERSED AND THE SILVER HALIDE EMULSION FORMED IS SUBSEQUENTLY CHEMICALLY SENSITIZED THE IMPROVEMENT WHICH COMPRISES RAISING THE PH OF THE SILVER HALIDE EMULSION TO A VALUE ABOVE 8 SOME TIME BETWEEN THE CONCLUSION OF THE WASHING OPERATION AND THE CHEMICAL SENSITIZATION.
 2. Method according to claim 1, wherein the said gelatin-derivative is phthaloyl gelatin.
 2. physical ripening of the silver halide grains,
 3. lowering the pH to bring about coagulation,
 3. Method according to claim 1, wherein the silver halide grain formation occurs in an aqueous solution of the said gelatin-derivative.
 4. Method according to claim 1, wherein the washed coagulum is redispersed under addition of water and normal gelatin.
 4. removal of the supernatant liquid and washing of the coagulum,
 5. redispersion of the washed coagulum under addition of water and gelatin,
 5. Method according to claim 1, wherein the pH of the silver halide emulsion is raised to a value above 8 after having redispersed the washed coagulum.
 6. Method according to claim 1, wherein immediately before coagulation the temperature of the physically ripened emulsion is adjusted to a value between about 10* and about 30*C.
 6. raising the pH of the redispersed coagulum to a value above 8 and keeping the emulsion at this value for some minutes,
 7. adjusting the pH to a value between about 6 and about 7,
 7. Method according to claim 1, wherein the coagulum is washed at least once with cold water.
 8. Method according to claim 7, wherein the pH of the first wash water is adjusted to the pH at which coagulation is effected.
 8. chemical sensitization of the emulsion to the optimum fog-sensitivity relation.
 9. Method according to claim 1, wherein the silver halide emulsion is a highly sensitive ammoniacal silver bromiodide emulsion.
 10. Method according to claim 1, comprising the steps of: 